The present invention relates to a printer or similar electrophotographic image forming apparatus for forming images on optical disks or similar synthetic resin sheets. More particularly, the present invention relates to an image forming apparatus for forming a toner image on a synthetic resin sheet while conveying the sheet with a conveyor, and causing a fixing device to fix the toner image on the sheet with heat.
Today, an electrophotographic image forming apparatus capable of forming attractive full-color images on, e.g., paper sheets and OHP (OverHead Projector) forms are extensively used. Further, there has been proposed in various forms an image forming apparatus of the type forming an image on one surface of an optical disk, e.g., a CD (Compact Disk), a CD-RW (CD ReWritable), an LD (Laser Disk), a DVD (Digital Versatile Disk) or similar synthetic resin sheet, e.g., on the protection layer surface of a CD. It has been customary with this type of image forming apparatus to use offset printing or screen printing. However, the problem with offset printing or screen printing is that a master corresponding to a desired image must be produced by an extra process beforehand. As a result, the apparatus lacks efficiency when producing many kinds of images or increases cost when producing a small number of images.
In light of the above, Japanese Patent Laid-Open Publication No. 5-212857, for example, proposes an electrophotographic label printer for optical disks operable in the same manner as the traditional image forming apparatus for paper sheets or similar recording media. The label printer does not need masters and therefore the extra process for producing them.
Generally, in an electrophotographic image forming system, a toner image formed on an image carrier is transferred to the surface of a synthetic resin sheet and then fixed on the sheet by heat. Such image transfer and fixation are effected with the sheet being conveyed by a pallet or similar holding member. An optical disk, for example, is thicker than a paper sheet and circular.
We have already proposed an image forming apparatus in which the surface of a synthetic resin sheet is resiliently displaceable relative to the circumferential surface of a transfer drum or similar image carrier. The surface of the sheet overlaps the circumference of the image carrier at the axis side of the image carrier when held in an unstressed position. At an image transfer position, the surface of the sheet contacts the image carrier and is resiliently displaced thereby. The sheet then presses itself against the image carrier due to the resulting restoring force, so that a pressure for image transfer acts between the sheet and the image carrier.
Likewise, the surface of the sheet is resiliently displaceable relative to the circumferential surface of a heat roller or similar fixing member. The surface of the sheet overlaps the circumference of the fixing member at the axis side of the fixing member when held in an unstressed position. At a fixing position, the surface of the sheet contacts the fixing member and is resiliently displaced thereby. The sheet then presses itself against the fixing member due to the resulting restoring force, so that a pressure for fixation acts between the sheet and the fixing member.
The pressure for image transfer or the pressure for fixation therefore varies with the amount of overlap of the surface of the sheet and the circumference of the image carrier or that of the fixing member, respectively. It follows that a preselected amount of overlap must be set up at each of the image transfer position and fixing position. In practice, however, the preselected amount of overlap is sometimes not set up due to irregularity in the configuration of parts and in assembly. An amount of overlap greater than the preselected one would aggravate an impact on the contact of the sheet with the image carrier or the fixing member and would thereby damage the image carrier or the fixing member. An amount of overlap smaller than the preselected one would bring about defective image transfer or defective fixation.
The image carrier, for example, contacts the circular sheet in the direction perpendicular to the direction in which the sheet is conveyed (direction of sheet transfer hereinafter). Therefore, the width over which the image carrier contacts the sheet being conveyed varies every moment. So long as the image transfer pressure acting on the transfer drum is constant, it increases for a unit width with a decrease in the width of the sheet contacting the transfer drum and vice versa. The image transfer pressure so varying with the width of the sheet adversely effects image formation. For example, the image transfer pressure causes a toner image to be partly lost if short or causes a toner image to remain on the transfer drum due to reverse transfer if excessive. This is also true with the fixing pressure. Specifically, the fixing pressure causes a toner image to come off due to short fixation if short or renders gloss irregular if excessive.
A difference between the moving speed of the surface of the sheet and the peripheral speed of the transfer drum or that of the fixing roller also adversely influences image formation. For example, if the moving speed of the sheet and the peripheral speed of the transfer drum are different, then an image is expanded or contacted. If the moving sheet of the sheet and the peripheral speed of the fixing roller are different, then an image is rubbed or gloss becomes irregular.
Further, when the holding member or the sheet carried thereon contacts the transfer drum at the image transfer position, the end corner of the former is apt to abut against and damage the latter. This is also likely to occur at the fixing position where the fixing roller is positioned.
Silicone oil or similar parting agent is often coated on the fixing roller in order to prevent toner from depositing on the roller. The parting agent is apt to deposit on the holding member and then deposit on the transfer drum during the next image formation. The parting agent deposited on the transfer drum obstructs the transfer of the toner to the drum, resulting in defective images.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 11-167312 and 11-305560.
It is therefore an object of the present invention to provide an image forming apparatus capable of electrophotographically forming an image on a synthetic resin sheet with a preselected image transfer pressure and a preselected fixing pressure, thereby insuring high quality images.
It is another object of the present invention to provide an image forming apparatus capable of forming high quality images by obviating a difference between the moving speed of a synthetic resin sheet and the peripheral speed of an image carrier or that of a fixing member.
It is yet another object of the present invention to provide an image forming apparatus capable of protecting an image carrier and a fixing member from damage.
It is a further object of the present invention to provide an image forming apparatus capable of preventing a parting agent from depositing on at least the image forming range of an image carrier.
An apparatus for forming an image on a synthetic resin sheet of the present invention includes an image carrier. A toner image forming device forms a toner image on the image carrier. A holding member holds the synthetic resin sheet on its surface that is resiliently displaceable when subjected to a force other than the weight of the sheet. A conveying device conveys the sheet held on the surface of the holding member along a preselected path. A transferring device transfers the toner image from the image carrier to the sheet being conveyed by the conveying device. A fixing device includes a fixing member for fixing the toner image transferred to the sheet. Rollers are mounted on at least one of the image carrier and fixing member at preselected positions for causing the surface of the holding member to be resiliently displaced such that the image surface of the sheet and the circumference of at least one of the image carrier and fixing member overlap each other by a preselected amount.